U.S. Pat. No. 4,212,701 discloses an improved procedure for preparing aluminum surfaces for adhesive bonding, which comprises etching the aluminum surface with an etching bath consisting essentially of a mixture of aqueous sulfuric acid and ferric sulfate. The procedure eliminates the pollution problem and toxic and carcinogenic fumes associated with the conventional procedures, wherein the aluminum is etched with an aqueous solution of sulfuric acid and sodium dichromate; and it produces aluminum surfaces, which when adhesively bonded, provide joint strengths comparable to those obtained using the standard sulfuric acid-sodium dichromate etch. In a further study of the process of U.S. Pat. No. 4,212,701, it was suggested that the ferric ion in the ferric sulfate-sulfuric acid etchant for aluminum functions by both retarding sulfuric acid attack on the surface and by oxidative attack of ferric ion on aluminum and copper (R. F. Wegman, D. W. Levi, K. M. Adelson and M. J. Bodnar, "The Function of the P2 Etch in Treating Aluminum Alloys for Adhesive Bonding", Proceedings, 29th National SAMPE Symposium, April 3-5, 1984, p. 273.